International Research Journal of Engineering and Technology (IRJET) Volume: 04 Issue: 02 | Feb -2017
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e-ISSN: 2395 -0056 p-ISSN: 2395-0072
Screening and extraction of heavy metals from anaerobically digested sewage sludge Samuel Wairiuko Ndiritu1, Dr. Charles Nzila2, Prof. Sitati Namango3 1Msc.
Scholar, Department of Chemical and Process Engineering, Moi University, Eldoret, Kenya Department of Manufacturing, Industrial and Textile Engineering, Moi University, Eldoret, Kenya 3Professor, Department of Chemical and Process Engineering, Moi University, Eldoret, Kenya
2Doctor,
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Abstract - Heavy metals extraction from anaerobically
impacts [2]. Heavy metals are harmful to both human and animals [3]. They are present in soluble form in the aqueous solution over a wide range of pH values and quite mobile in the natural environment [4]. The removal of heavy metal from sludge before disposal or application to farmland is a necessary step to achieve a more safe sludge usage or disposal [5]. Currently, methods such as membrane filtration, ion exchange, reverse osmosis and electrochemical extraction used for heavy metal removal from sewage sludge are quite expensive [6]. Therefore, efforts have to be directed toward finding strategies that are less expensive and less damaging to soil properties.
digested sewage sludge is an important step towards achieving a safe usage of the large amounts of sewage sludge on agricultural land. The extraction reduces heavy metals accumulation in soil and their bioaccumulation in plants, animals and humans through food-chain thus reducing both human health problems and adverse environmental impacts. Chemical extraction of heavy metals from anaerobically digested sewage sludge using Citric acid was studied using the full factorial design. The three factors considered were pH, Hydrogen peroxide dosage, and extraction time at two different levels: pH (3.0 and 5.0), hydrogen peroxide dosage (1g/l and 5g/l), and extraction time (1day and 10 days) with the objective of obtaining the models for the heavy metals extractions. The results were analyzed statistically using the Student’s t-test, analysis of variance, F-test, to define the most important process variables affecting the heavy metal extraction efficiency. Lead had the highest extraction at 99.90%, followed by Nickel at 99.87%, Copper at 99.80% and Zinc at 99.68%. The most significant effect was ascribed to extraction time followed by Hydrogen peroxide dosage and the interaction of the two. The pH effect and the interaction between pH and time also had an influence in extraction efficiency of heavy metals. There was no significant interaction between pH and Hydrogen peroxide dosage in the extraction of heavy metals Zn, Pb, Ni and Cu under tested conditions.
Ideally the sludge coming out from a bioleaching system would present reduced amounts of both toxic metals and pathogenic organisms [7]. Although it has the advantage of heavy metals recovery [8], its major disadvantage is the sensitivity of microorganisms to high metals toxicity levels [9]. [10], highlighted that Ion exchange is a versatile process which accommodates metal ion concentration variations and reasonable changes in flow rate without deterioration in performance however it has major limitations of high capital and operation costs when the heavy metals concentrations are high and the sensitivity to particles present. It has been observed that electrodialysis has main shortcomings of limited strength and high cost of the cation selective membrane. In addition high power consumption makes its industrial applications rare [11]. In adsorption method the fact that the adsorbents can be reactivated and reused is a major advantage of this method. However, treating large quantities of waste water would require large beds which will require a large inventory of expensive adsorbents leading to high capital cost. In addition adsorbents progressively deteriorate in capacity as number of cycles increases and as a result the large quantities of spent adsorbents containing heavy metals may be considered a hazardous waste [12]. The limitations associated with reverse osmosis involve the sensitivity of the membrane. Organics as well as other impurities precipitate lead causing membrane fouling. It is therefore necessary to have a consistent composition of the influent waste stream which is hard to achieve in a waste water treatment plant. In addition the process also requires elevated pressures that drive up the operating costs due to pumping [13]. The application of chemical extraction as a part of the treatment is a feasible
Key Words: Sewage sludge, Heavy metals, chemical leaching, Citric acid, Hydrogen peroxide
1. INTRODUCTION Wastewater treatment and the management of the solids it produces are global issues, with growing challenges, that must address the concerns of all of the stakeholders, including the facility administrators and operators, the regulators, the politicians, the scientific community, the wastewater generators, the taxpayers and the general public [1]. Sewage sludge contains high heavy metal concentrations in addition to useful high quantities of nutrients and organic matter needed for plant growth. Heavy metals can accumulate in soil and in plants when sludge is applied as fertilizer. The potential accumulation of heavy metals in human tissues and biomagnification through the food-chain create both human health problems and environmental
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